class Solution {
public:
    int maxProfit(vector<int>& prices) {
      int len = prices.size();
      if(len == 0||len == 1) return 0;
      vector<vector<int> >dp(len,vector<int>(5,0));
      dp[0][0] = 0;
      dp[0][1] = -prices[0];
      dp[0][2] = 0;
      dp[0][3] = -prices[0];
      dp[0][4] = 0;
      for(int i = 1;i < len;i++){
        dp[i][0] = dp[i-1][0];
        dp[i][1] = max(dp[i-1][1],dp[i-1][0]-prices[i]);
        dp[i][2] = max(dp[i-1][2],dp[i-1][1]+prices[i]);
        dp[i][3] = max(dp[i-1][3],dp[i-1][2]-prices[i]);
        dp[i][4] = max(dp[i-1][4],dp[i-1][3]+prices[i]);
      }
      return dp[len-1][4];
    }
};
